Digital communications systems commonly employ special data patterns for synchronization purposes. These carefully-selected n-bit patterns, which are called unique words, are imbedded in the transmittal data to provide a mechanism for aligning the receive-side signal processing functions with the received data.
Early work on the use of unique words in TDMA applications is described in the paper entitled "Unique Word Detection in Digital Burst Communications," by W. Schrempp et al., IEEE Transactions on Communications Technology, Vol. COM-16, No. 4, August 1968, pp. 597-605, which paper analyzed unique word detection in terms of false detection probability, missed detection probability and unique word correlation properties. The paper also described a basic unique word detector as consisting of an n-bit shift register, n modulo-2 adders, and a summation and threshold network.
Since the unique word is generally known to occur periodically, false detection performance may be enhanced by gating the detector output with an aperture signal (or detection window) which is active in the region of expected unique word occurrence, as described in the above-referenced paper. Because only those detections which coincide with the aperture gate are considered, false detections occurring outside the aperture limits are rejected.
However, the most computationally intensive part of the unique word detection process in the initial acquisition, or open aperture mode of operation. In this operating mode, little or no information is usually available as to the location of the unique word, so all possibilities are typically considered in an exhaustive search.
The article by B. H. Warner, Jr., entitled "Build High-Speed Sync-Pattern Detectors," Electronic Design, Oct. 11, 1975, pp. 82-85, described two approaches used successfully is unique word detectors operating at speeds above 50 Mbit/s. The first approach used standard emitter-coupled logic (ECL) functions to implement the detector as an n-bit shift register, an n-bit adder tree, and a digital comparator. In the second approach, high-speed programmable read-only memories (PROMs) were used to implement the adder function as a code converter.
Both of these high-speed unique word detectors were designed to perform a complete unique word detection for every data clock cycle. At the beginning of each clock cycle, the serial bit stream was shifted by one bit and the potential unique word was checked against the detection criteria. In addition to the difficulties inherent in implementing the detection function at high speed, these approaches become more complicated when the unique word is spread throughout the data, rather than being clustered in one portion of the frame.
At more moderate data rates, however, it is quite practical to implement a real-time unique word detector using various commercially available correlation chips. Such a detector can readily perform an exhaustive search until the unique word is located.
At the opposite extreme from the high-speed hardware detector is an approach in which an amount of data equal to an entire frame (i.e., the amount of data from one unique word to the next) is stored in memory. The stored data may then be slowly searched for the unique word in an exhaustive manner. New data which are received during the unique word search are kept track of by means of a counter, so that the desired unique word reference point can be established relative to the current data stream once the unique word is found. This approach is particularly well-suited for low data rate applications employing a microprocessor or digital signal processor (DSP) chip, where the unique word detection algorithm and other signal processing can be conveniently performed via software.
Any unique word detection technique has some possibility of failure due to the chance of either a false detection or a missed detection. The probability of either of these occurrences is usually kept negligibly low by the choice of the unique word size and the allowable number of errors. However, each of the conventional methods described above involves an exhaustive search in a predetermined manner until a detection occurs.